Internal guard for polar checks
Examples
af <- example_aniframe(n_obs = 3, n_individuals = 1, n_keypoints = 1)
# Passes silently when the coordinate system matches
try(ensure_is_polar(af))
#> Error in ensure_coordinate_system(data, "polar", "polar") :
#> This aniframe is not in a polar coordinate system.
#> ℹ coordinate_system is "cartesian_2d".
#> ℹ Convert the coordinates first; anispace has the transformations.